<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    header{
        line-height: 50px;
        height: 50px;
        background-color: brown;
    }
    footer{
        line-height: 50px;
        height: 50px;
        background-color: cornflowerblue;
    }
    #content{
        float: left;
        width: 100%;
        background-color: cyan;
        margin: 0 auto;
        height: 600px;
    }
    #left{
        position: relative;
        float: left;
        height: 600px;
        width: 200px;
        background-color: darkgrey;
        margin-left: -100%;
        right: 200px;
    }
    #right{
        position: relative;
        float: right;
        height: 600px;
        width: 300px;
        background-color: green;
        margin-left: -100%;
        left: 300px;
    }
    #main{
        width: 100%;
        padding-left: 200px;
        padding-right: 300px;
        box-sizing:border-box;
    }
</style>
<body>
    <header></header>
    <main id="main">
        <div id="content"></div>
        <div id="left"></div>
        <div id ="right"></div>
    </main>
    <footer style="clear: both;"></footer>
</body>
</html>